1. Field of the Invention
An aspect of the invention relates to a density and viscosity sensor. The invention further relates to a density and viscosity measuring method.
Such a density and viscosity sensor and measuring method find a particular, though non exclusive, application in measuring density and viscosity of fluids in harsh environment including high pressure conditions, high or low temperature conditions, or a combination of both conditions. Such harsh environments may be found with respect to measurement applications in desert region, in arctic region, in deep sea zone, in subterranean zone, etc. . . .
2. Description of the Related Art
The document EP 1 698 880 describes a density and viscosity sensor for measuring density and viscosity of fluid, the sensor comprising a resonating element arranged to be immersed in the fluid, an actuating/detecting element coupled to the resonating element, and a connector for coupling to the actuating/detecting element. The sensor further comprises a housing defining a chamber isolated from the fluid, the housing comprising an area of reduced thickness defining a membrane separating the chamber from the fluid. The actuating/detecting element is positioned within the chamber so as to be isolated from the fluid and mechanically coupled to the membrane. The resonating element arranged to be immersed in the fluid is mechanically coupled to the membrane. The membrane has a thickness enabling transfer of mechanical vibration between the actuating/detecting element and the resonating element.
The document US 20100154546 describes an apparatus for determining and/or monitoring at least one process variable of a medium. The apparatus includes at least one mechanically oscillatable membrane, which has a plurality of natural eigenmodes; and at least one driving/receiving unit, which excites the membrane to execute mechanical oscillations and/or which receives mechanical oscillations from the membrane. The driving/receiving unit and the membrane are embodied and matched to one another in such a manner, that the membrane executes only mechanical oscillations, which correspond to modes, which lie above the fundamental mode of the membrane. In an embodiment, the driving/receiving unit includes at least one piezoelectric element. The piezoelectric element has at least two separate regions, and the piezoelectric element is arranged and connected with the membrane in such a manner, that the two separate regions of the piezoelectric element excite, each, a section of the membrane to execute mechanical oscillations. If the two regions of the piezoelectric element are supplied either with counter-phase (equal polarization), or with equal phase (opposite polarization), alternating voltage signals, then each region executes a different oscillation, i.e. one region contracts (the thickness decreases) and the other region expands (the thickness increases). This leads to the fact that also the corresponding sections of the membrane, which, in each case, preferably, reside above different regions of the piezoelectric element, also, in each case, execute different oscillations. Thus, in such an apparatus, the two regions of the piezoelectric element supplied with appropriate voltage signals are used to select a desired mode of oscillation of the membrane which itself constitute the resonating element of the apparatus.
Such density and viscosity sensors or apparatuses are well adapted for measurements in harsh environment. However, they are not satisfactory because the vibration modes of the membrane create stationary acoustic waves that interfere with the measurements. This results in measurements accuracy issues.